docs: def help_completion_fields(self):
          """ Return valid field names.
          """
          for name, field in sorted(engine.FieldDefinition.FIELDS.items()):
              if issubclass(field._matcher, matching.BoolFilter):
                  yield "%s=no" % (name,)
                  yield "%s=yes" % (name,)
                  continue
              elif issubclass(field._matcher, matching.PatternFilter):
                  yield "%s=" % (name,)
                  yield "%s=/" % (name,)
                  yield "%s=?" % (name,)
                  yield "%s=\"'*'\"" % (name,)
                  continue
              elif issubclass(field._matcher, matching.NumericFilterBase):
                  for i in range(10):
                      yield "%s=%d" % (name, i)
              else:
                  yield "%s=" % (name,)
  
              yield r"%s=+" % (name,)
              yield r"%s=-" % (name,)
  
          yield "custom_"
          yield "kind_"